Until recently, the prior art had not made yttrium silicon oxynitrides independently so that they could be used as a powder additive for a variety of subsequent chemical processes. Oxynitrides, if formed at all, were formed as a comingled byproduct of heating, such as by hot pressing a ternary system of silicon nitride, and two oxides used as hot pressing aids (see U.S. Pat. Nos. 4,102,698; 4,341,874; and 4,350,771). In each of these patents, a compact of the ternary powder system is sintered to a relatively high density during which secondary phases form, which may include oxynitrides. Of course, the process was not aimed at producing an oxynitride that could be extracted for other uses. The high density of the resulting product made it very difficult to separate out any oxynitrides that had been formed and even more difficult to convert the high density sintered product into a reground powder. But, more importantly, the purity and type of second phases produced as a result of such sintering could not be accurately controlled, and the chemistry constraints imposed by the presence of excess silicon nitride inhibited the formation of desired or selected yttrium silicon oxynitrides.
A recent effort (see U.S. Pat. No. 4,331,771) has been made to produce bonded silicon oxynitrides for utility as a structural material. In this effort, silicon powder and oxides are reacted in the presence of an oxygen/nitrogen atmosphere. The method is designed to provide for a strong and dense oxynitride body required by the utilization of the body, which strength can be obtained only by the presence of impure substances in the oxynitride body.
What is needed is a method by which a low density, pure form of the oxynitride can be made economically for other uses with minimal regrinding. There is a need for a method by which stoichiometric amounts of the ingredients needed for a specific oxynitride can be chemically reacted to form the specific compound without byproducts and contaminating chemical reactions.